Epilepsies comprise a diverse collection of disorders that affect an estimated 1-4% of the population in the United States. Epileptic seizures, which are generally self limiting, are the outward manifestation of excessive and/or hypersynchronous abnormal activity of neurons in the cerebral cortex. The behavioral features of a seizure reflect function of the portion of the cortex where the hyper activity is occurring. Generalized seizures, which appear to involve the entire brain simultaneously, can result in the loss of consciousness only and are then called absence seizures (previously referred to as “petit mal”). Alternatively, the generalized seizure may result in a convulsion with tonic-clonic contractions of the muscles (“grand mall” seizure). Some types of seizures, partial seizures, begin in one part of the brain and remain local. An individual suffering a partial seizure may remain conscious. If the individual loses consciousness, the seizure is referred to as a complex partial seizure. Current drug therapies operate either by regulating postsynaptic responses to neurotransmitter or by blocking presynaptic transmitter release machinery at a step prior to exocytosis (e.g., by inhibiting calcium and sodium channels). Such therapies can reduce seizure frequency in the majority of patients but it is estimated that only about forty percent are free of seizures despite optimal treatment. Unfortunately, currently available drug treatments are often associated with onerous side effects because—in addition to preventing seizures—they affect normal brain function as well.
Synapsins are the most abundant proteins associated with synaptic vesicles (De Camilli et al, Annu. Rev. Cell Biol. 6:433-60. (1990)). They are present in all of the most common types of presynaptic terminals: those that are activated primarily by action potentials (APs) (De Camilli et al, Annu. Rev. Cell Biol. 6:433-60 (1990)). Because they are extensively phosphorylated in an activity dependent fashion, and reversibly tether reserve vesicles to cytoskeletal elements within presynaptic terminals (Greengard et al, Science 259:780 (1993)), they have long been thought to be involved in regulating the synaptic vesicle exo/endocytic cycle (De Camilli et al, Annu. Rev. Cell Biol. 6:433-60 (1990)). However, despite extensive molecular and physiological investigation, an identification of their role in neurotransmission has remained elusive—partly because techniques have only recently been developed that allow the individual rate-limiting steps in the vesicle cycle to be studied in isolation.
Several key experiments have implicated synapsins in the regulation of processes that afford synapses the ability to transmit signals during extended periods of heavy use. Synaptic strength temporarily weakens extensively during repetitive use, a phenomenon known as short-term depression (Zucker, Annu. Rev. Neurosci. 12:13-31 (1989)). Synapses with disrupted synapsin function (Pieribone et al, Nature 375:493-7 (1995), Hilfiker et al, Nat. Neurosci. 1:29-35 (1998), Humeau et al, J. Neurosci. 21:4195-206 (2001)), or synapses of mutant mice that lack synapsins (Rosahl et al, Cell 75:661-70 (1993), Rosahl et al, Nature 375:488-93 (1995), Li et al Proc. Nati. Acad. Sci. USA 92:9235-9 (1995), Terada et al, J. Cell Biol. 145:1039-48 (1999)), depress more quickly than do normal synapses, even though the basic cell biological machinery that underlies the exo/endocytic cycle seems to be intact (Rosahl et al, Cell 75: 661-70 (1993), Rosahl et al, Nature 375:488-93 (1995), Ryan et al, J. Cell Biol. 134:1219-27 (1996)). The precise kinetic role played by synapsins in short-term depression remains obscure.
Several of the kinetic components that underlie short-term depression by limiting the rate at which synaptic vesicles are prepared for release have been identified at excitatory synapses of the hippocampus (Stevens et al . Proc. Natl. Acad. Sci. USA 92:846 (1995), Stevens et al, Neuron 24:1017 (1999)). A typical synaptic terminal contains hundreds of vesicles laden with neurotransmitter but at any moment at most only a few percent of those are docked at the active zone, readily available to undergo exocytosis (Schikorski et al, J. Neurosci. 17:5858 (1997), Schikorski et al, Nat. Neurosci. 4:391 (2001)). These release-ready vesicles supply the transmitter used for intercellular signaling. During periods of heavy use, the readily releasable pool (RRP) of vesicles is quickly exhausted and the synaptic strength weakens because further transmission is only possible when new vesicles replace the spent ones within the pool.
The present invention is based, at least in part, on studies that have resulted in the elucidation of the biological mechanism responsible for synaptic fatigue. As a result of this understanding of mechanism, it has now been appreciated that agents that selectively reduce the size of the actively recycling vesicle pool at excitatory synapses can be used to prevent incipient seizures from developing into epilepsy. Such agents can be expected to avoid the onerous side effects inherent in currently available treatments as they should not affect synaptic function during periods of ordinary use.